Description from Flora of China
Chamaenerion hirsutum (Linnaeus) Scopoli; Epilobium hirsutum var. laetum Wallich ex C. B. Clarke; E. hirsutum var. sericeum Bentham ex C. B. Clarke; E. hirsutum var. tomentosum (Ventenat) Boissier; E. hirsutum var. villosum (Thunberg) H. Hara; E. tomentosum Ventenat; E. velutinum Nevski (1937), nom. illeg. superfl., not H. Léveillé (1916); E. villosum Thunberg.
Herbs robust, perennial, sometimes woody near base with long, thick, ropelike hypogeal stolons often terminating in a rosette of leaves. Stems 25-120(-250) cm tall, much branched in upper half, densely villous pubescent, with short glandular hairs especially on inflorescence, rarely sparsely pubescent, or rarely densely white tomentose. Leaves sessile and clasping stem; cauline blade lanceolate-elliptic to narrowly obovate or elliptic, rarely very narrowly lanceolate, 4-12(-23) × 0.3-4(-5) cm, both surfaces villous, very rarely glabrescent, base subcuneate and clasping, margin serrulate with 20-50 teeth per side, apex acute to acuminate. Inflorescence and flowers erect. Sepals 6-12 mm, often keeled. Petals bright pink to dark purple, 8-20 mm. Stigma deeply 4-lobed. Capsules 2.5-9 cm, pubescent or rarely glabrescent; pedicels 0.5-2 cm. Seeds dark brown, 0.8-1.2 mm, coarsely papillose, with inconspicuous chalazal collar; coma tawny or dull white, detaching easily. Fl. Jun-Aug, fr. Jul-Sep. 2n = 36.
This is an extremely widespread and variable species that spreads aggressively by vegetative growth in wet habitats.
Populations from Xinjiang tend to have strikingly tomentose pubescence, unlike plants from other regions, but the pattern of variability for entire species obscures these differences, so no subdivision is recognized.
Wet places near streams, ditches, marshes, gravel or sandy beds of rivers, roadsides; (200-)500-2000 m in N China, (100-)500-2800(-3500) m in SW China. Anhui, Gansu, Guangdong, Guizhou, Hebei, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Nei Mongol, Ningxia, Shaanxi, Shandong, Shanxi, Sichuan, Xinjiang, Xizang, Yunnan, Zhejiang [Afghanistan, India, Japan, Korea, Mongolia, Nepal, Pakistan, Russia; widespread in Africa, SW Asia, Europe, and naturalized in North America].